Field-symmetric beam detector for semiconductors

ABSTRACT

An alignment mark and pattern is disclosed for use on semiconductor substrates which are to be patterned in an electron lithography machine. The detector includes two interleaved N-well portions mounted on a P-substrate. The interleaved &#34;fingers&#34; of the N-well portions are spaced to provide narrow gaps which are approximately the width of a projected electron beam. When the beam is located within the gap (or gaps) the projection is in alignment.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

FIELD OF THE INVENTION

This invention provides an improved alignment mark and pattern for useon semiconductor substrates which are to be patterned in a 1:1,electron-projection lithography machine.

BACKGROUND OF THE INVENTION

A 1:1 electron-projection lithography machine permits the projection ofan emitted pattern of low-energy electrons from a masked photocathodeonto a substrate (e.g. a semiconductor wafer) as a high energy, focused,patterned beam at unity magnification, thereby permitting the transferof the pattern to an electron sensitive material on the surface of thesubstrate which can subsequently be caused to selectively alter thesubstrate. It is possible to control the magnification by a small amountwithout introducing significant distortion into the projected pattern,even over a large surface such as an eight inch diameter wafer.

In using such a machine to manufacture fine geometry products such asintegrated circuits having 0.1 micron features, it is generallynecessary to repeat the lithography process with successive beampatterns aligned with previously created structures to an accuracy ofone-fourth to one-tenth of the size of the smallest features. Thus, touse 0.1 micron features, it is desirable to align successive patterns towithin 0.01 micron. The procedure used in the past for alignment ofpatterns in such machines has been to use two or more small structuresresulting from the first photocathode pattern, which lie substantiallyat the ends of a diameter of the substrate, as electron detectors, andto provide a matching beam pattern on the succeeding photocathodes.

Typically an electron detector on the substrate is more sensitive toelectrons than the surrounding portion of the substrate so signals canbe generated which are used to align the small pattern beams with theircorresponding detectors.

The prior art generally provides a system in which a beam is movedacross a detector. The maximum response of the detector as the beam ismoved across it denotes the condition of alignment. In order to obtain aposition indication from which a useful centering signal can be derivedit is necessary to provide a modulation of the beam position.Unfortunately, this also modulates the position of the entire patterncausing it to lose resolution. Therefore, the modulation must be removedafter alignment, and the alignment cannot be monitored during the actualprinting of the pattern. In addition, changes in the size of thedetectors during intervening processing steps will reduce thesensitivity to position by widening the peak of the response.

PRIOR PATENTED ART

A search of the prior patent art revealed the U.S. Patents describedbelow.

U.S. Pat. No. 4,992,394 issued to Kostelak, Jr. et al is concerned withthe enhancement of registration marks for the purpose of improving theirdetectability, as by an electron beam in electron beam lithography,whereby the accuracy of alignment is improved. This patent does notresolve the problems associated with modulation of beam position, i.e.,loss of resolution as well as inability to monitor alignment duringactual printing of the pattern. Kostelak, Jr. et al makes no mention ofthe essence of the present invention, i.e., employment of the symmetricfield as the actual "mark" to effect beam alignment.

U.S. Pat. No. 4,981,529 issued to Tsujita is concerned with the problemof erroneous recognition of alignment marks resulting from theinterrupted flow of resist by obstructions provided around the alignmentmarks to prevent uniform covering by the resist film. Tsujita solvesthis problem by a certain geometric arrangement of certain alignmentmarks such that the peak intensity levels of diffraction light obtainedby the alignment marks are constant. This is unrelated to the teachingof the present invention which uses a symmetric field.

U.S. Pat. No. 4,546,534 issued to Nicholas is concerned with the problemof alignment of the doped region of a semiconductor device, referred toas a parasitic thick field stopper, and the overlying insulating layerpattern, a thick field oxide in the case of IGFETs. The patented methodalleged to solve the problem involves two separate exposure steps withthe same mask being used for both exposures. Nicholas acknowledges thatthe steps of his method are quite conventional and does not usesymmetric fields as marks to effect beam alignment.

U.S. Pat. No. 4,238,685 issued to Tischer relate to a method forpositioning a mask pattern with respect to a substrate. The patentedmethod involves a relative displacement of the x-ray source whilemaintaining a rigid association between the mask and the substrate, or arelative displacement of the mask and the substrate together whilemaintaining the ray source fixed. There is no suggestion whatsoever ofthe present invention procedure of aligning the beam from the ray sourcewith symmetric fields as marks.

U.S. Pat. No. 4,109,029 issued to Ozdemir et al involves sequentialformation of first and second masks having openings positionallyreferenced to first and second alignment marks, respectively. Incontrast, the present invention utilizes marks in the substrate asdetectors, while effecting alignment of the beam with symmetric fields.

U.S. Pat. No. 3,745,358 issued to Firtz et al involves beam scanning ofalignment holes, whereas the present invention involves beam alignmentwith symmetric fields as marks.

SUMMARY OF THE INVENTION

In summary, this invention is a system and method for detecting andpositioning electron beams on semiconductor substrates during electronprojection lithography by using symmetric fields as marks. The inventionprovides for a signal of one polarity if the beam is misaligned in onedirection, a signal of the opposite polarity if misaligned in theopposite direction, and a zero signal when properly aligned. The systemincludes two interleaved N-wells mounted on a p substrate. Theinterleaved "fingers" of the N-wells are spaced to provide narrow gapswhich are approximately the width of a projected electron beam. When thebeam is located within the gap the projection is in alignment. In apractical case, a plurality of beams is projected in a pattern identicalto the pattern of the gaps.

In accomplishing the foregoing and related objects, the invention alsoprovides for the detection and positioning of a narrow electron beamthat is sufficiently energetic to create electron-hole pairs in asemiconductor.

The beam has an elongated cross-section that lies in a planesubstantially perpendicular to the direction of beam propagation, andthe current density of the beam is symmetric with respect to thecenterline of the elongated cross-section.

The detector for the beam is substantially perpendicular to thedirection of beam propagation, and includes a first or substrateelectrode that is elongated about a centerline which is coincidable witha substantial portion of the centerline of the beam cross-section.

The first or substrate electrode of the detector is locally andsubstantially symmetric with respect to a generally cylindrical surfacegenerated about the electrode centerline in the propagation direction. Apair of second electrodes is spaced from and substantially parallel tothe first or substrate electrode, and from one another. The secondelectrodes are locally symmetric with respect to the generallycylindrical substrate surface.

At least one substantially electrically carrier-free material occupiesspace adjoining the electrode substrate and touches each of the secondelectrodes. The material is locally symmetric about the generallycylindrical substrate surface.

An electric field is established within the electrically carrier-freematerial, and is locally mirror-symmetric with respect to the generalcylindrical surface in the vicinity of the coincidable portion of thecenterline of the first electrode, and sufficiently large to separateand collect hole-electron pairs generated within the material by thebeam.

As a result, a first current may be deleted and measured flowing betweenthe first or substrate electrode and one of the second electrodes, and asecond current may be deleted and measured flowing between the first orsecond substrate electrode and the second electrode by virtue ofseparation, within the field, of electron-hole pairs generated withinthe electrically carrier-free material.

The construction of the portions of the electrodes and beam lyingoutside coincidable portions of the centerlines is such that only smallchanges in currents can result from any small lack of coincidence in thedirection of elongation. However, the spacings are large enough thatprocessing beyond that required to construct the detector will not makethe spacings unusably small. The centerlines are positioned with respectto one another so that their coincidence, and the direction of any smalllack of coincidence in a direction perpendicular to the propagationdirection and the direction of elongation, may be determined from themagnitude and sign of the difference between the first current and thesecond current when the coincidable portions of the center lines of thebeam and first electrode are coarsely aligned. Positioning can reduceany small lack of coincidence in the perpendicular direction.

The detector can include a first semiconductor electrode of a firstconductivity type and a semiconductor substrate of anopposite-conductivity type. The electrically carrier-free material has acarrier-depleted region between the semiconductor of one conductivitytype and the semiconductor of opposite conductivity type. The detectoralso can be formed with electrodes that are metallic conductors.

The detector can include a multiplicity of individual detectors withcoincidable portions of centerlines substantially parallel and lyingnear a plane parallel to the cross-section. The detectors can bedisplaced from one another in a direction substantially orthogonal tothe centerlines, such that the coincidable portions of the centerlinesfor the detectors are simultaneously coincidable with the coincidableportions of the centerlines of a corresponding multiplicity of beams,with the first electrodes connected in common, and the second electrodesseparately connected in common.

The detector can detect currents using resistors connected from thesecond electrodes to a common voltage source formed from a semiconductorof the same conductivity type as the second electrodes, and voltages canbe sensed between the common voltage source and the second electrodes.

The detection of currents can be by resistors connected from therespective common connections of the second electrodes to a commonvoltage source and formed from a semiconductor material of the sameconductivity type as the second electrodes. Voltage is sensed betweenthe common voltage source and the second electrodes.

An alignment pattern can be used on semiconductor substrates which areto be patterned in an electron lithography machine. The detector caninclude two interleaved N-well portions mounted on a P-substrate, withinterleaved "fingers" of the N-well portions spaced to provide narrowgaps which are approximately the width of a projected electron beam.When the beam is located within the gap (or gaps) the projection is inalignment. The electrodes are sufficiently thin that the beam canpenetrate through them.

In a multiplicity of such detectors, the coincidable centerlines of thedetectors are substantially parallel and lie nearby in a plane parallelto the cross-section such that the centerlines of the detectors aresimultaneously coincidable with the centerlines of a correspondingmultiplicity of beams.

For a pair of detectors lying near to one another having coincidablecenterlines simultaneously coincidable with corresponding beamcenterlines, and their coincidable centerlines lying near the plane,with the coincidable centerlines of one of the pair substantiallyorthogonal to those of the other.

For detecting and positioning a propagated electron beam pattern on asemiconductor substrate, the beam has a pattern comprising at least onenarrow electron beam elongated along a centerline that is perpendicularto the direction of propagation of the beam. The current density in thebeam is symmetric with respect to the center line. A first electrodecomprises a semiconductor of one polarity; second spaced electrodescomprise semiconductors of opposite polarity to the first electrode andare positioned on the first electrode, with the space between the secondand third electrodes having a centerline coincidable with the centerlineof the electron beam.

A signal of one indication is extracted when the electron beam strikesone of the second electrodes, and the first electrode, and there isanother indication when the beam strikes the second electrodes edge. Thebeam is properly aligned when the centerline of the beam patterncoincides with the centerline of the gap and the indications aresubstantially equal.

In a detector for detecting and positioning an electron beam patternthat is larger than its width, electron projection lithography is on asemiconductor a P-substrate below elongated N-well fingers on thesubstrate, each finger having a substantially straight edge and spacedapart by a substantially constant width gap approximately the width ofthe electron beam and having a centerline coincidable with the centerline of the beam.

A signal of one indication is extracted when the beam strikes one N-wellfinger, or near its edge, and another there is another indication whenthe beam strikes another N-well finger, or near its edge. The beam isproperly aligned when the centerline of the beam pattern coincides withthe centerline of the gap, and the indications are substantially equal.

A common voltage supply biases each of the N-wells through first andsecond resistors, respectively. The electron beam pattern edges and gapsare all made by electron projection lithography, using the same mask foreach to insure that the centerlines of the gap and the electron beampattern coincide.

A detector for detecting and positioning an electron beam pattern on asemiconductor substrate during electron projection lithography includesa P-substrate; first and second elongated N-well portions with eachelongated interleaved fingers having spaced apart edges forming gaps ofsubstantially constant width. A plurality of parallel gaps are formedbetween fingers, and the beam pattern has a geometry corresponding toalternate gaps. A signal of one indication is extracted when the beampattern strikes one of the N-well portions, or near the edges of oneN-well, and there is another indication when the beam pattern strikesthe other of the N-wells portions, or near its edges. The beam patternis properly aligned when the centerline of the beam pattern coincideswith the centerline of the gaps, and the indications are substantiallyequal.

The electron beam pattern, the edges and gaps are all made by electronprojection lithography, using the same mask for each to insure that thecenter lines of the gap and the electron beam pattern coincide.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparentafter considering the following description taken in conjunction withthe illustrative embodiment in the accompanying drawings, in which:

FIG. 1 is illustrative of the performance of the prior art;

FIGS. 2-4 show successively lower magnification plan views of a set ofdetectors constructed in accordance with this invention;

FIG. 5 is a cross section FIG. 2;

FIG. 6 is a curve showing the performance of the detector;

FIGS. 7a-7d show the method of making the photocathodes such that thecenterlines of beams and gaps match exactly at the central response, andFIG. 7 is an illustration that shows the position and placement of then-well, beam and steering of the photocathodes of FIGS. 7a-7d;

FIGS. 8a to 8d illustrate the invention schematically; and

FIG. 9 is a schematic showing of an alternate construction of adetector.

DESCRIPTION OF A PREFERRED EMBODIMENT

The prior art of FIG. 1 shows a misaligned beam 10, a detector 12, andthe response 14 of the detector as the beam is moved across it. Notethat the condition of alignment is marked by a maximum peak in theresponse at 16. In order to obtain a position indication from which auseful centering signal can be derived it is necessary to provide amodulation of the beam position, i.e., the beam must move across thedetector. Unfortunately, this also modulates the position of the entirepattern causing it to lose resolution. Therefore, the modulation must beremoved after alignment, and the alignment cannot be monitored duringthe actual printing of the pattern. In addition, changes in the size ofthe detectors during intervening processing steps will reduce thesensitivity to position by widening the peak of the response.

In the prior art, signals from detectors located near the ends of adiameter can be used to correct X alignment, Y alignment and rotationalalignment, and to correct magnification errors, provided the detectorsat each end are sensitive to both X and Y displacements. This is thecase with the detector mark shown if position modulation is provided inboth directions. Simultaneous centering of all beams on thecorresponding marks must be attained.

In accordance with this invention, FIGS. 2-4 show successively lowermagnification plan views fingers for of a set of X detector 2 (see FIG.4). FIG. 2 shows part of a rotated view of a detector which is locatednear the end of a diameter of a substrate. A similar set of detectors(not illustrated in FIG. 4) is located near the other end of thediameter. Each detector X and Y comprises two symmetric sets of,interleaved "fingers" of N-wells e.g. 20 for V1x and 22 for V2x mountedin or on a substrate 24 (better seen in the cross section of FIG. 5,where the beam is shown positioned between finger 20 of V1x and finger22 of V2x) with gaps 26 and 28 between the interleaved N-wells fingers.

Referring to FIG. 4, where a complete X-Y sensitive mark pair is shown,the system includes a Y detector and an X detector, both powered by acommon voltage supply 31 through resistors 32 and 34, and 36 and 38,respectively. The electrons of the beam (indicated by position markerson FIGS. 2-4) preferably have sufficient energy to penetrate through theN-well layer so that some signal is generated whenever a beam isstriking the N-well near a piece of an N-well edge to create electronhole pairs within collection range of the N-well edge.

Wherever the electron beam traverses a semiconductor, it generates manyelectron-hole pairs. The common supply voltage biases the N-well to forma depletion region between the N-type and P-type regions. In order toget the best resolution the beam should also penetrate far enough intothe undepleted P-type portion of the substrate to reduce the number ofstraggling electrons which can reenter the depletion region. Under theseconditions the holes will be collected by the substrate and theelectrons will be collected primarily by the nearest part of the N-well.If the beam strikes entirely though N-well, then all the current fromthe pairs generated in the depletion region beneath the well will becollected by that piece of the N-well. However, if the beam strikesexactly centered on the gap between the two pieces of N-well edge, theywill collect the current equally as long as the voltage on the twopieces of N-well edges remain equal. This will be the case when theresistors formed by the narrow portions of the N-wells (R1_(x) andR2_(x)) in FIG. 4, for example) are equal. Provided that these resistorsare low enough in resistance that the voltage drops in them caused bythe currents are small compared to the voltage supply, the symmetricfield in the gap between the pieces of N-well edge will be substantiallymaintained even when the beam is off center in the gap. The actual marksare the alternate symmetric fields between adjacent "fingers" of theN-well.

Refer now to FIG. 8a, where the detector is shown in its simplest form.That is, where the N-wells of the detector are shown with only twofingers F1 and F2, and with an elongated electron beam B1 located in thegap between the fingers. In the position shown, the voltages developedby the system are equal to zero and the beam is aligned in thehorizontal (or X) direction.

In the arrangement shown in FIGS. 8b to 8d, each of the N-wells areshown with two fingers F1, F3 and F2, F4, respectively. A beam patternconsisting of elongated narrow beams B1 and B2 having the identicalshape and size as the gaps between the fingers F1 and F2, and F3 and F4,respectively, is projected onto the detector. If the beams B1 and B2 arelocated in the gaps as shown in FIG. 8b, the system is in alignment andthe voltage developed is equal to zero. However, if the beam is out ofthe gaps, as shown in FIG. 8c, voltages will be developed on the V2xN-well. On the other hand if the beams are on the V1x well, adifferential voltage of the opposite polarity is developed. In eithercase, the system is known to be out of alignment.

It is noted that in FIGS. 8a to 8b, the length of the beam is equal tothe length of the gap. While this feature is not essential to theoperation of the overall system, it does provide course alignment in theY axis. Note, for example in FIG. 8d, the beam is not in a gap, but ispositioned on the V1x well. By aligning the beams between the 2 wells,the system will be coarsely aligned on the Y-axis. Fine alignment willbe completed with the Y detector.

It should be noted that it is not necessary that the fingers be exactlyparallel. It is important that projected beam have a substantiallyidentical shape as the gap between pairs of fingers into which the beamis projected. It should also be noted, that the number of fingers ineach detector is chosen as a function of required sensitivity. One ortwo gaps, as shown in FIGS. 8a to 8d, may not, in a practical case,provide sufficient detector sensitivity.

The differential voltage characteristic (V2_(x) -V1_(x)) versusmisalignment of such a detector is shown in FIG. 6. The only effect ofthe small field asymmetry mentioned above would be a slight broadeningof the characteristic around the exact alignment position. The exactbalance would remain at the exact alignment point. A similar broadeningeffect will result from a symmetric change in the size of the gap orbeam by some processing step.

The characteristic shown is local around the position where all the beamcenterlines of a detector are aligned with their corresponding gaps'centerlines (note the gaps are really all parts of a single gap sincethe topology of the entire N-well pattern in FIG. 4 is that of acircle). Providing the beams of a detector are all spaced equally, asimilar but smaller amplitude signal will be found when a smaller numberof beams are aligned. This characteristic makes for ease of coarsepositioning since the successive coincidences can simply be counted asthe set of beams of a detector is swept from outside the detector to thecentral alignment position. The crossing of beams with the gapscorresponding to blocked beams has an inverted characteristic. Note thata small rotational misalignment does not change the location of thecentral largest response, but only its amplitude. Small in this casemeans small enough that most of each beam is striking its correspondinggap when the center of the beam is centered on its gap. That is one ofthe reasons for choosing the length of each beam to be no more than fourhundred times it width; mechanical initial alignment of substraterotation can easily be done to well within that tolerance, for example,0.001 inch at each end of the diameter. The widths of the beams and gapsare chosen to be greater than the minimum geometry in order to insureagainst lithographic faults which might short N-well across a gap, aftermany processing steps. That also reduces the necessity for having exactevenness of spacing of the gaps; in fact, small variation in therelative rotations and spacings of the gaps and their correspondingbeams can be used intentionally to reduce all of the responses but theexact-matching correct ones.

The response of the detector as a whole is much like the derivative ofan autocorrelation function between gaps and beams. The distance fromgap to gap is chosen to be great enough to insure a negligibledistortion of the symmetry of the field at each gap. This will also givea clean separation of successive responses as the beams sweep across thedetectors. A further consideration is that the voltage drops in the two"fingers" of a gap should be small. This last criterion is not criticalsince such drops would produce field asymmetries which would mostlycancel at the correct alignment position, slightly reducing theamplitude of the response.

FIG. 9 shows a variation where the three electrodes 40, 41, and 42 aremetallic conductors and such that there is physical symmetry of thecurrent density in the beam about its centerline and of the electrodesabout the centerline of the middle electrode. All three are on, or in, acarrier-free substrate 43 which might be almost any insulator providedthat enough voltage can be applied from the outer two to the middle onefor hole-electron pairs (generated by a beam which could eitherpenetrate the electrodes or be wider than the center electrode) to see alocally anti-symmetric field which will separate them so as to provideequal currents to the outer two when the beam is aligned. The importantpoints are the physical mirror-symmetry which creates a mirror-symmetryof the electric field from a symmetric potential (scalar) field.

In all of the above, it has been assumed that the centerlines of beamsand gaps match exactly at the central response. This can be insured bymaking the photocathodes of both by electron projection lithography froma common pattern but using opposite "tones". This is illustrated inFIGS. 7A to 7D. The original pattern could be that of the meander whichforms the gap plus the surrounding field as in FIG. 7A. This pattern isput onto two photocathodes. One of the photocathodes is then used forthe first pattern in the lithography process, from which the N-wellpattern is generated, while the other is altered (by a process whichneed not be very accurate), as in FIG. 7B, so as to remove the bends ofthe gap pattern and alternate beam patterns, and printed with a "tone"to produce beam pattern rather than N-well pattern, for example,negative and positive resists, respectively.

The surrounding fields should also be blocked (not shown). Alternatebeams are removed since they would cancel the differential responses. Atthe correct alignment, each beam should have the correct phase in itsresponse. In the blocking process illustrated, each beam length isreduced enough so that at alignment condition, the beams are well awayfrom the asymmetric fields where the gap changes direction, and centeredin both directions. However, as pointed out with respect to thesimplified embodiments illustrated in FIG. 8a to 8d, it is to beunderstood that the beam length may be made equal to the gap length.

Note that a detector retains some discrimination of position even whenthe beams are seriously out of alignment in the direction parallel tothe length of the beams. The portion of the beams which overlap one ofthe end areas simply cause a constant differential signal. The overlaps,of course, disappear as both X and Y beams come to central alignment.

Not shown in FIG. 5 are overlying layers such as a resist and aconductive layer deposited on top of it to prevent charge accumulationon the surface. Charge accumulation on the surface causes beam patterndistortions. Such a conductive layer shields the beams from any fieldsused in the detectors. Overlying layers, particularly P-type orintrinsic semiconductor, will merely provide additional sensitive volumewhile maintaining the field-symmetry of the gaps. Slight surface leakageshould have no significant effect on operation.

The various connecting areas (V1, V2, Common Supply, etc.) must haveprobes to them for signal extraction and voltage supply. These probesmust be positioned such as not to introduce irreproducible distortion inthe electric field of the machine since that would affect the alignment.One way to accomplish that is simply to put the probe pads at amoderately large distance (a few millimeters) from the detectors asindicated by the unspecified length of the connecting part of the N-wellpattern. Such a location permits the shielding of these connecting areasfrom processing steps, and permits low resolution removal of connectionobstructing layers. Connection must be made to the substrate or P-wellfor return current.

The overall sensitivity of the detector is controllable by adjusting thecommon supply voltage. This has two effects, both of which are due tothe change in the thickness of the depletion layer: 1) the sensitivevolume changes, and 2) the resistance per square of the sensingresistors changes. These are aiding effects. The effects of a moderatechange in size of the N-well pattern (which narrows the gap) can also becompensated as long as even a partial gap remains. The illustratedlayout is not, but can and should be capacitively balanced. Theconnection pads should be heavily doped for good connections.

Electron-projection lithography is well known in the prior art. It isknown that if a plane surface in a vacuum is caused to emit a pattern oflow energy electrons by masking and illuminating it while a parallelplane surface is maintained at a high potential relative to the firstsurface, then the transit time of all the emitted electrons to thesecond surface will be substantially identical. If a magnetic fieldperpendicular to the two planes is set at such a value that the periodof the cyclotron resonance of electrons in that field is an integralfraction of the transit time than all electrons leaving a point on thefirst surface will return to a point on the second surface directlyfacing the point on the first surface, creating an image in high energyelectrons suitable for exposing a pattern in a resist layer on thesecond surface. The second surface may be a silicon wafer, e.g., or maybe a blank which is intended for use as another mask. The resolution ofthe image is comparable to that of electron microscopes having similaraccelerating voltages.

Low-distortion control of the position or orientation--relative to apreexisting structure on the second surface--at which the electronpattern strikes can be obtained by mechanical motion or, for smallmotions of the pattern, by applying small uniform magnetic fieldsperpendicular to the main field. Small changes in magnification ordemagnification of the projected pattern can also be made by producing asmall non-uniformity in the main magnetic field.

The advantages of this invention are its superior accuracy of alignment,the ability to verify and maintain alignment during the actual printingof the overall substrate pattern and the availability of signals whichcan be used in coarse positioning without losing sensitivity for finepositioning. The new features are the use of symmetric fields as"marks", the provision of sensitivity and size adjustments, and thetopology which permits the inclusion of multiple relatively short beamsby sacrificing roughly half of the beams which could be place in orderto phase the signal from all remaining beams coherently.

Many alternatives should be obvious to persons skilled in thesemiconductor art. For example, N-type and P-type materials may beinterchange. Dimension can be altered within relatively wide limits. TheN-well can be on the surface of the P-type material rather than beembedded in it. There may be silicon dioxide in the gaps, or partiallyin the gaps, rather than semiconductor etc. providing the locallymirror-symmetric fields are maintained. The symmetry is mirrorsymmetric, i.e. symmetric about a plane, physically and for thepotential, making the field (which is the gradient of the potential)anti-symmetric.

Moreover, while the embodiments shown in FIGS. 2-8 show a detectorhaving first and second wells having straight "fingers" which areinterleaved in parallel relationship, it is to be understood that adetector may be constructed wherein the fingers are made of variousshapes, so long as the parallel relationship between them is maintained,and so long as the beam is symmetrical with the gap.

In addition, as shown in FIG. 9, there may be three conductors 40, 41and 42 mounted on a substrate 43. The important thing is that there area pair of collection volumes 44 and 45 established by two electrodes 40and 42 at one potential, and a third electrode 41 centered in the gapbetween the first two at a second potential, so as to create a symmetricpair of charge collection volumes. An electron beam of substantiallyuniform current density directed toward the detector is such that whenthe centerline of the beam coincides with the centerline of the thirdelectrode 41, equal numbers of charge pairs are generated in the twosensitive volumes.

Clearly, many other modifications and variations of the presentinvention are possible in light of the above teachings and it istherefore understood, that within the inventive scope of the inventiveconcept, the invention may be practiced otherwise than specificallyclaimed. For example, while each of the claims specifies a P-substrateand N-wells, although not as desirable in its application, anN-substrate and P-wells are deemed to be the full equivalent.

What is claimed is:
 1. A detector for detecting the extent of alignment of an electron beam, comprising:a first electrode; a plurality of second electrodes spaced from one another to form a gap between each pair of adjoining second electrodes; and means for biasing the first and second electrodes to create a carrier-free region between said first electrode and each second electrode; whereby said electron beam striking one of said second electrodes, or said gap between said second electrodes, cause a differential flow of current having a magnitude and direction depending upon the position of said beam relative to said second electrodes.
 2. A detector according to claim 1 wherein said first electrode is a semiconductor of one conductivity type and said second electrodes are semiconductors of an opposite conductivity type.
 3. A detector according to claim 1 wherein the first and second electrodes are all metallic.
 4. A detector according to claim 1 wherein said first electrode is a doped substrate upon which said second electrodes are mounted.
 5. A detector according to claim 1 wherein each second electrode includes a multiplicity of elongate fingers which are interleaved with a multiplicity of fingers of another second electrode.
 6. A detector according to claim 5 wherein the fingers of said second electrode are parallel to one another.
 7. A detector according to claim 1 wherein the biasing means comprises a substrate upon which the second electrodes are mounted and spaced from one another.
 8. A plurality of detectors, each according to claim 1 wherein the first electrodes of said detectors and their second electrodes are respectively connected in common.
 9. A pair of detectors, each according to claim 1 wherein the second electrodes of one pair are orthogonal to the second electrodes of the other pair.
 10. A pair of detectors, each according to claim 1 wherein the second electrodes of said pair are respectively connected in common, with the second electrodes of one pair orthogonal to the second electrodes of the other pair.
 11. A detector according to claim 1 wherein the means for biasing comprises respective resistors extending to a common voltage source formed from a semiconductor of the same conductivity type as said second electrodes.
 12. The method of detecting the extent of alignment of an electron beam, comprising the steps of:(a) providing a first electrode positioned relative to a plurality of second electrodes having a gap therebetween each pair thereof; (b) biasing the first and second electrodes to produce a carrier-free region in a substrate between said first electrode and each second electrode; and (c) causing said electron beam to strike said substrate through said second electrode, or said gap between said second electrodes, whereby a differential flow of current from affected said second electrodes has a magnitude and direction dependent upon the position of said beam relative to said second electrodes.
 13. The method of claim 12 wherein said beam is caused to have sufficient energy to penetrate through said second electrode.
 14. The method of claim 12 wherein the biasing of said first and second electrodes produces a depletion layer between said first electrode and each second electrode.
 15. The method of claim 14 wherein said beam is caused to have sufficient energy to penetrate beyond said depletion layer to reduce the incidence of electrons which enter said depletion layer.
 16. The method of claim 12 wherein said gap has a length and said beam is caused to have a length which is substantially that of said gap.
 17. The method of claim 12 wherein sensitivity is increased by increasing the number of said second electrodes, and said first and second electrodes are biased through resistors providing a substantially smaller voltage drop than the source of the biasing.
 18. The method of claim 12, further comprising the step of depositing a resist conductive layer on the surface of the detector or to inhibit charge accumulation on the surface of the detector.
 19. The method of claim 12, further comprising the step of positioning probe pads for signal extraction to avoid electric field distortion from the gaps of said second electrodes.
 20. The method of detecting and assuring alignment of an electron beam with a prescribed gap of a detector, comprising the steps of:(a) making photocathodes by electron beam projection lithography starting with a common gap pattern produced in one tone; (b) adapting said gap pattern for said beam using an opposite tone to remove alternate gaps; whereby the centerlines of beams and gaps match at central response. 